1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly, to optimization of write power.
2. Description of the Related Art
An optical disk apparatus for recording data on a rewritable optical disk, such as a CD−RW, DVD−RW, or DVD-RAM, has already been developed. The optical disk apparatus records data after having optimized write power. The followings are steps for optimizing write power.
(1) The optical disk apparatus records test data in a predetermined area [test area: PCA (Power Calibration Area)] of an optical disk.
(2) The optical disk apparatus overwrites test data at the same location by changing write power in a plurality of levels.
(3) The optical disk apparatus reproduces the test data after overwriting, to thereby measure jitter for each level of write power employed.
(4) The optical disk apparatus sets the write power, at which jitter becomes minimum, as optimum write power.
When data are overwritten by radiating a laser beam onto the optical disk, the optical disk apparatus superimposes erase power and write power on read power of the laser beam. The data that have already been recorded are erased with the erase power, which is applied before the write power. New data are recorded with subsequent write power. An optical disk of phase change type is taken as an example. A recording film is gently heated with the erase power and then gently cooled to change the recording film from an amorphous state to a crystalline state, thereby erasing data. The recording film is rapidly heated and cooled with the write power to change the recording film from the crystalline state to the amorphous state, thereby recording data. Write power optimum for overwriting operation is determined through the forgoing optimization steps, and the erase power is determined by multiplying the thus-optimized write power by a constant ratio.
However, in the optimization steps, there may arise a case where jitter of the overwritten test data varies according to the magnitude of an RF amplitude of the test data that have already been recorded even when the same write power is employed during overwriting operation. When comparison can be drawn between a case where the test data that have already been recorded have a large RF amplitude and a case where the test data have a small RF amplitude, greater erase and write power is required in the case where the test data have the large RF amplitude. For this reason, when the test data that have already been recorded have a large RF amplitude, jitter of the overwritten test data becomes deteriorated. Thus, the method—in which test data are overwritten by merely changing the write power in a plurality of levels and optimum write power is selected on the basis of the jitter of the overwritten test data—involves a problem of optimum write power not always being true optimum write power that can effect overwriting regardless of an RF amplitude of the data that have already been recorded.
In the optical disk such as DVD-RAM, the RF amplitude of data becomes saturated as the write power is increased, and no substantial change arises in jitter even when the write power is increased further. By utilization of this phenomenon, write power at which an RF amplitude becomes saturated is searched, and the thus-searched write power can be taken as optimum write power. However, in reality, a change in jitter has essentially disappeared before the RF amplitude becomes saturated. Consequently, the method for selecting write power at which the RF amplitude is saturated as optimum write power results in optimum write power being set so as to become higher than original optimum write power; that is, write power at which best jitter and a best error rate are achieved. Such high write power deteriorates durability, such as the number of rewriting operations. Consequently, even when write power at which an RF amplitude becomes saturated is merely searched and the thus-searched write power is set as optimum write power, the write power is effective for a certain number of writing operations. However, in view of durability, the method is not the best setting method.